Maximizing plyometric training for adolescents: a meta-analysis of ground contact frequency and overall intervention time on jumping ability: a systematic review and meta-analysis

Plyometric training boosts adolescents' jumping ability, crucial for athletic success and health. However, the best total ground contact frequency (TGCF) and overall intervention time (OIT) for these exercises remain unclear. This meta-analysis aims to identify optimal TGCF and OIT in plyometric training for adolescents, focusing on countermovement jump (CMJ) and squat jump (SJ) outcomes. This systematic review encompassed five databases and included 38 studies with 50 randomized controlled experiments and 3347 participants. We used the Cochrane risk assessment tool for study quality and Review Manager 5.4 for data analysis. The current meta-analysis incorporated a total of 38 studies, comprising 50 sets of randomized controlled trials, to investigate the influence of different TGCFs and OITs on plyometric training. The Cochrane risk assessment tool indicated that all the included studies were classified as low risk. Various TGCFs in plyometric training positively affected CMJ and SJ heights in adolescents. The TGCF of less than 900 was ideal for enhancing CMJ, whereas more than 1400 was effective for SJ. The optimal OIT was 400–600 min, specifically, 500–600 min for CMJ and 400–500 min for SJ. Plyometric training improves jumping ability in adolescents. Lower ground contact frequency (< 900 contacts) enhances CMJ, while higher ground contact frequency (> 1400 contacts) is more effective for SJ. Optimal intervention time ranges from 400 to 600 min, with 500 to 600 min benefiting CMJ and 400 to 500 min improving SJ.


Information and research methods
The present systematic review and meta-analysis followed the PRISMA guidelines, and the protocol has been registered in PROSPERO (ID: CRD42023446372).

Search strategy
The present systematic review and meta-analysis conducted searches in databases including PubMed, Web of Science, Scopus, ProQuest, and China National Knowledge Infrastructure (CNKI).The search process is illustrated in Fig. 1 (PubMed was used as an example).The search time frame extended from the beginning of records in each database to February 6th, 2023.To ensure the accuracy of the search, two researchers (D.L and L.C) cross-checked the search keywords.If there were any disagreements on keyword selection between the two researchers, a third researcher (Z.H) would make the final decision.If necessary, manual searches would be conducted to supplement the literature.

Study selection
The literature inclusion criteria for this meta-analysis were based on the participants, intervention, comparison, outcome, and study (PICOS) format of evidence-based medicine.
The inclusion criteria were as follows: (1) the study participants were adolescents aged 10-19 years 21 ; (2) the experimental group underwent plyometric training as an intervention, followed by their participation in specialized training (such as football, basketball, volleyball, etc.) or regular physical education classes, which were identical to those provided to the control group.The control group did not receive the intervention of plyometric training but engaged in activities similar to the experimental group, including specialized training or regular physical education classes; (3) the control group served as a blank control, receiving no intervention of plyometric training, but participating solely in specialized training or regular physical education classes; (4) all studies were conducted on rigid ground to avoid interference from different training surfaces; (5) the outcome measures were jump performance indicators, including CMJ and SJ; (6) all studies should include one of the following variables: TGCF or OIT; (7) the study design was a randomized controlled trial (RCT).
The exclusion criteria were as follows: (1) non-randomized controlled trials, self-controlled trials, and randomized crossover trials; (2) inability to obtain study data; (3) conference papers, reviews, and comments; (4)  unable to obtain full-text articles; (5) outside the age range of adolescents.
A total of 38 studies that met the inclusion criteria were included in the analysis.The detailed inclusion and exclusion process is shown in the figure below (Fig. 2).

Data extraction
To avoid duplications, all literature from the various databases was imported into the EndnoteX9 software for deduplication.After confirming all the final literature included in the analysis, two researchers (C.L and H.Z) extracted the data into a Microsoft Excel spreadsheet.The extracted data included author's name, publication year, characteristics of the subjects (such as age or maturation stage, gender, sample size), pre-and post-test

Statistical analysis
The data analysis was conducted using the Review Manager 5.4 software (Review Manager, The Nordic Cochrane Centre, Copenhagen, Denmark).To ensure the objectivity and persuasiveness of the study, the present metaanalysis and subgroup analysis were conducted with data from no less than two groups 22 .All units of measurement were converted to a common unit (e.g., all units of length were converted to centimeters).Therefore, the weighted mean difference (WMD) with a 95% confidence interval was used as the overall effect size indicator in the final analysis.The heterogeneity index I 2 was used to evaluate the heterogeneity of the studies.I 2 values lower than 25%, between 25 and 75%, and higher than 75% are considered to have negligible, moderate, and high heterogeneity, respectively 23 .If the heterogeneity of the studies was less than 25%, a fixed-effect model was used for outcome indicator analysis.If the heterogeneity was greater than 25%, a random-effect model was used for outcome indicator analysis 8 .P < 0.05 was considered statistically significant.
Since there are currently no studies that classify different TGCF, this study used an artificial classification method to present the research results more persuasively.The aim was to ensure that each subgroup contained a comparable number of studies, thereby increasing the comparability and persuasiveness among the subgroups.Specifically, different TGCFs were divided into three groups: low ground contact frequency (LGCF), medium ground contact frequency (MGCF), and high ground contact frequency (HGCF).
LGCF included experiments with a TGCF of less than 900, MGCF included experiments with a TGCF between 900 and 1400, and HGCF included experiments with a TGCF greater than 1400.In addition, OIT was classified by every 100 min as a subgroup, for example, 200-300 min as a subgroup.This finer classification aimed to provide a more detailed analysis.If the number of articles in each subgroup was less than 2, the subgroup would be canceled and not included in the analysis.

Ethics approval
The study has been registered in the International Prospective Register of Systematic Reviews (PROSPERO: CRD42023446372).

Study characteristics
According to the PRISMA reporting guidelines, 38 studies comprising 50 experimental groups were included in this meta-analysis.The studies involved 3347 adolescent participants aged 10-19 years, including those with specialized training and those without.The intervention in the experimental group was a plyometric training, while the control group received only specialized training without any other training intervention.The majority of the studies had an intervention time of 20-40 min, an intervention period of 6-10 weeks, and an intervention frequency of twice a week (Table 1).

Risk of bias in the included articles
The Cochrane risk of bias assessment tool was used to assess the quality of all the literature included in this metaanalysis.All the studies included in this analysis were randomized controlled trials, with 8 studies reporting allocation concealment and 6 studies implementing blinding of the researchers and participants.All the studies included in this meta-analysis were rated as low risk of bias (Fig. 3).

Meta-analysis results
The impact of plyometric training with varying ground contact frequencies on jumping ability in adolescents A total of 28 studies comprising 36 experimental groups and 982 participants were included in this metaanalysis to evaluate the impact of plyometric training with varying ground contact frequencies on CMJ height in adolescents (Fig. 4).The overall effect size showed a significant positive effect of plyometric training on CMJ height in adolescents [MD = 2.62, 95% CI (1.99 A total of 14 studies comprising 19 experimental groups and 537 participants were included in this metaanalysis to evaluate the impact of plyometric training with varying ground contact frequencies on SJ height in adolescents (Fig. 5).The overall effect size showed a significant positive effect of plyometric training on SJ height

Reporting bias
The symmetrical distribution in the funnel plots did not suggest the presence of publication bias (Fig. 8).

The relationship between TGCF and jumping performance
The results of the meta-analysis indicate that plyometric training is an effective method for improving jumping performance, which supports the view that plyometric training can improve lower limb explosiveness 8,24,25 .Subgroup analyses indicate that different TGCTs induce specific adaptations in each type of jump.For the CMJ, it appears that LGCF is more beneficial, followed by MGCF and HGCF.In the case of SJ, HGCF seems to yield better results, followed by MGCF and LGCF.Similar to the results of most meta-analyses on the effects of plyometric training on jumping ability 8,9,25,26 , this study also found that plyometric training is an effective method for improving jumping performance in adolescents.Mark et al. 27 conducted a similar experimental study to this one, where they divided all participants into low, moderate, and high-intensity training groups based on the different ground contact frequencies per week.Their results showed that low-intensity training can achieve similar effects as moderate and high-intensity training in the reactive strength index (RSI) measure.In contrast to Mark et al. 27 , this study classified the TGCF of the intervention, rather than just the frequency per week, and used CMJ and SJ as outcome measures to investigate the effects of different ground contact frequencies on jumping ability.www.nature.com/scientificreports/ The primary reason for the improvement in jumping ability following plyometric training is the enhancement of central nervous system adaptation and muscle strength and explosiveness.Specifically, neural adaptation is characterized by changes in muscle activation strategy, which involve increased activation of agonist muscles and decreased activation of antagonist muscles 28 , or improvements in muscle excitability resulting from lengthening and shortening 29 .The improvement in muscle strength and explosiveness is mainly due to changes in muscle structure, such as a reduction in muscle fascicle angle and an increase in muscle fascicle length 30,31 , and changes in the stiffness of various elastic components, such as the plantar flexor-tendon complex [32][33][34][35] .These neurophysiological changes can enhance the efficiency of energy storage and release during the stretch-shortening cycle (SSC), ultimately resulting in improved jump performance 36 .
However, for the results of this meta-analysis, it appears that LGCF can better promote the improvement of CMJ ability in adolescents.Currently, there is not enough evidence to explain this phenomenon, and the physiological mechanisms underlying the effects of plyometric training with different TGCF on CMJ ability are not yet clear.Therefore, the following summarizes the potential mechanisms by which plyometric training with LGCF can better induce improvements in CMJ ability in adolescents: (1) CMJ is a widely used method for testing lower limb muscle strength and explosiveness, characterized by utilizing the SSC effect during jumping to generate higher jump heights 37 .The SSC effect not only involves the intrinsic biomechanical mechanisms of muscles but is also closely related to adaptive regulation of the nervous system 38 .In the SSC process, the muscle can activate more motor units for rapid contraction after being rapidly stretched, requiring the neuromuscular system to respond sensitively and accurately to signals of muscle lengthening 39 .
LGCF may be more conducive to inducing neural adaptation, thereby further enhancing the SSC effect 38 Adolescents are at a peak period of growth and development, which makes them more susceptible to overfatigue and sports injuries 40 .Compared to adults, adolescents have a greater neural load, making it difficult to effectively regulate the SSC process 41 .Therefore, plyometric training with LGCF can effectively reduce the neural load, promote neural monitoring and control of SSC 42,43 .Reducing training intensity appropriately can reduce neural fatigue, allowing for greater attention to SSC regulation, thus enhancing neural adaptation 44,45 .
(3) During high-intensity rapid muscle contractions, a large amount of lactate is produced, leading to muscle soreness and fatigue 46 .The accumulation of lactate in the muscle can inhibit muscle contraction ability, reducing muscle strength and explosiveness 47 .In addition, rapid muscle contraction requires a significant amount of energy, including ATP, creatine phosphate, and glycogen, among others.Energy depletion is also an important factor contributing to muscle fatigue [47][48][49][50] .Therefore, LGCF can reduce muscle lactate accumulation and fatigue, helping athletes maintain stable jumping performance [51][52][53] .
In contrast to the CMJ indicator, plyometric training with HGCF can better induce improvements in SJ ability.SJ is also a test method for measuring lower limb muscle strength and explosiveness, characterized by involving only concentric force during jumping 54,55 .More TGCFs in plyometric training can better induce improvements in maximal strength and explosiveness, thereby improving adolescent SJ ability.The following are the related reasons for this phenomenon: www.nature.com/scientificreports/ (1) When muscles are stimulated by nerve impulses, the release and reuptake of intracellular calcium ions (Ca2 +) are important regulatory mechanisms for muscle contraction and relaxation 56 .After extensive training involving repeated contractions and relaxations, the muscle's ability to release and reuptake Ca2 + can be improved.This, in turn, enhances the generation of muscle tension and relaxation speed, and helps the muscle adapt to higher loads and stronger stimuli, thereby enhancing muscle strength and endurance 50,[57][58][59] .(2) With an increase in training frequency, motor neurons will connect to appropriate muscle fibers more frequently and make the synthesis and release of neurotransmitters more efficient 50,[60][61][62] .This leads to clearer transmission of nerve impulses to the muscles, thus activating more muscle fibers 63 .

The relationship between OIT and jumping performance
The results of this study indicate that the OIT for plyometric training is more reasonable in the range of 400-600 min for improving the jumping ability of adolescents.Subgroup analysis shows that for the CMJ index, the OIT for plyometric training can achieve the best training effect in the range of 500-600 min.As for the SJ index, the training effect is better with a OIT of 400-500 min.Despite many meta-analyses on the effect of plyometric training on the jumping ability of adolescents, there has been no study summarizing time recommendations for plyometric training in adolescents 8,9,25,26 .Therefore, there is currently no clear recommendation for the most suitable OIT for plyometric training in adolescents.Negra et al. 64   OIT is one of the important factors affecting the effect of plyometric training.Too long OIT may lead to athlete fatigue, overtraining, sports injuries, and even a decrease in training effect 65,66 .Conversely, too short OIT may result in poor training effect, failure to fully stimulate athlete potential, and limit the development of athletic performance 67 .Therefore, a reasonable OIT is crucial for optimizing the training effect, especially for adolescents.The importance of a reasonable OIT is reflected in the following aspects: (1) A reasonable OIT for plyometric training can provide adolescents with sufficient training volume, which is crucial for the development of the neural and muscular systems and for improving jumping performance.
Studies have shown that appropriate training volume can gradually help adolescents adapt to training, enhance the reaction speed and coordination of the neural-muscular system 44,68 .In terms of the neural system, appropriate training can improve the activation efficiency of motor units and the learning efficiency of movement patterns 69 .In terms of the muscular system, appropriate training can enhance muscle strength, explosive power, and elasticity, and improve the coordination between the neural and muscular systems 44,68 .These changes and improvements in the neural and muscular systems can enhance the neural-muscular reaction speed and coordination of adolescents, thereby improving their jumping ability 70,71 (2) Plyometric training positively impacts adolescent growth and development.Training duration and intensity play a crucial role in shaping the body and physiological systems 72,73 .During adolescence, growth hormone levels increase, stimulating the secretion of insulin-like growth factor 1 (IGF-1).This hormone promotes the growth of bones, muscles, and ligaments 74,75

Future research lines
Moving forward, we will strive to collect more experimental research related to adolescent plyometric training, particularly studies that closely associate TGCF and OIT, as well as movement intensity, with intervention periods.We hope that through this approach, we can provide more accurate and comprehensive data and conclusions to inform both the theory and practice of plyometric training.

Conclusion and practical applications
In conclusion, plyometric training has been proven effective for enhancing the jumping ability of adolescents.This meta-analysis highlights the importance of two key variables, namely TGCF and OIT, in influencing training outcomes.
The findings suggest that plyometric training with a LGCF (less than 900 contacts) is beneficial for improving CMJ ability in adolescents.On the other hand, plyometric training with a HGCF (more than 1400 contacts) is more effective for enhancing SJ ability.Additionally, the optimal OIT for improving jumping ability through plyometric training is within the range of 400-600 min.Specifically, an OIT of 500-600 min yields the best results for CMJ ability, while a training time of 400-500 min is more ideal for SJ performance.
Based on these findings, the following practical applications can be recommended: (1) Plyometric training is highly recommended for jump training in adolescents due to its effectiveness in enhancing their jumping ability (2) The determination of TGCF should be based on the training objectives.To improve CMJ ability, it is recommended to limit the TGCF below 900.For those aiming to improve SJ ability, a TGCF exceeding 1400 can be selected (3) OIT is a significant factor that affects training outcomes.For optimal improvement in CMJ ability, a total training time of 500-600 min is recommended.On the other hand, for better SJ performance, a training time of 400-500 min is more suitable.

Figure 2 .
Figure 2. PRISMA flow chart for inclusion and exclusion of studies.

Figure 3 .
Figure 3. Judgments about each risk-of-bias item for each included study and risk-of-bias item presented as percentages across all included studies.

Figure 4 .
Figure 4. Forest plot of the effect of plyometric training with varying ground contact frequencies on CMJ height in adolescents.

Figure 5 .
Figure 5. Forest plot of the effect of plyometric training with varying ground contact frequencies on SJ height in adolescents.

Figure 6 .
Figure 6.Forest plot of the effect of plyometric training with different overall intervention times on CMJ height in adolescents.
conducted a similar experimental study to explore the time effect of plyometric training on lower limb explosive power in adolescents with different total training weeks.The research team conducted four tests on the plyometric training group at baseline, week 4, week 8, and week 12.The results showed that the jumping ability of the plyometric training group improved significantly at week 12.Although Negra et al. 64 study compared the training differences of plyometric training with different training weeks, the OIT, i.e. the training time per week and the OIT, was not further refined due to the long time intervals between weeks 4, 8, and 12. Currently, there is no study that categorizes plyometric training with different OITs into levels, so it is unclear how long plyometric training should be considered a short or long OIT for adolescents.Therefore, to further explore the effect of plyometric training with different OITs on the jumping ability of adolescents, this study further subdivided the OIT into subgroups of 100 min each, in order to more accurately investigate the potential

Figure 7 .
Figure 7. Forest plot of the effect of plyometric training with different overall intervention times on SJ height in adolescents.

Table 1 .
Characteristics of study participants.PT plyometric training, CON control group, CMJ countermovement jump, SJ squat jump, TGCF total ground contact frequency, OIT overall intervention time, PE physical education, min minute, PPT progressed plyometric training, NPPT non-progressed plyometric training, FIXED fixed drop-box height, OPT optimal drop-box height, After plyometric training applied after soccer practice, Before plyometric training applied before soccer practice, early tanner stage maturation of 1-3, late tanner stage maturation of 4-5.

and future research lines Limitations Based
on extensive experimental research on plyometric training for adolescents, this study conducted an indepth statistical analysis of TGCF and OIT involved in plyometric training.However, we must acknowledge that due to the uniqueness of each experiment, we were unable to effectively correlate TGCF and OIT with intervention periods.Additionally, we recognize that the types of training movements have an important impact on training intensity and effects.However, in this study, we failed to quantify the intensity of various training movements involved in plyometric training, which may have affected our comprehensive understanding of the training effects.